The present invention relates generally to preforms for use in blow molding containers formed of polymeric resins. The invention particularly relates to such preforms having more than one layer of resin.
Preforms having more than one layer of resin material have been used in blow molding operations to form containers having multi-layered walls. The layered preforms can be formed by co-extrusion or co-injection of multiple polymer streams of resin into a single mold. Such co-injection processes are sometimes difficult to control and are subject to layer thickness variation as a result of variations in control parameters. A series of injection processes has also been employed to form a layered preform in a series of molds, the preform being transferred from mold to mold in the process of adding each new layer. The thickness build up of the preform using such a process translates into longer cooling times since cooling of the newly injected layer of plastic can only be achieved through contact of one side of the layer of resin. Any of these processes can be used to form a layered preform in which recycled resin forms an outside layer of the preform while virgin resin forms an inside layer of the preform, the inside layer forming the portion of the container most likely to experience contact with foods, beverages, drugs, cosmetics or the like within the container. For the purposes of this invention, all resin not suitable for food contact under part 177 of title 21 of the United States Code of Federal Regulations is considered as the equivalent to recycled resin.
It has also been suggested to independently mold two or more layers in separate molds and to thereafter form a preform by assembling a series of layers together. This method has the advantages of direct control of each layer of thickness since it is formed in a separate mold specifically fashioned for that layer where thickness tolerances can be carefully controlled. The process is also advantageous in that each layer is very thin and subject to cooling from both sides and therefore the production of each layer can be comparatively rapid. Since the layers can be simultaneously formed in separate molds, this formation time, even when added to the assembly time, can yield faster production for a preform over that experienced for molding a single or multiple layer preform in a single fixed mold. However, the subsequent handling of such an assembled preform has, on occasion, proved difficult as the assembled layers of the preform have a tendency to de-laminate, partially or entirely, prior to the blowing operation.